As is mentioned in this new post on the Symfony Finland site, the functionality allowing the Twig templating engine to perform Angular 2 Universal Rendering.

Angular is a household name when it comes to JavaScript frameworks. They’ve been going through a major transition since the announcement of Angular 2. In the latest development Angular 2 will be getting PHP / Twig support for isomorphic rendering.

[...] So the Angular and the Drupal development teams have started working together to bring Universal Rendering to PHP and Twig. The details remain murky on how exactly this will be implemented (using a Node.js server or a PHP library), but a PHP library (not an extension written in C) would be the easiest option for developers.

He points out that there’s also a chance that the result could be coupled to Drupal (at least at first) but hopes are that it will end up as a generic component usable in any system using Twig. The post also includes links where you can find out more information about the topic of "Isomorphic Rendering" too.